3.1 How does mercury accumulate in organisms?

The harmful effects that different forms of mercury can have
on living things are greatly influenced by
bioaccumulation (build up
inside an organism) and
biomagnification (build up
along the food chain).

This tighter binding leads
methylmercury to build up
(biomagnify) much more than
other forms of mercury along the food chain, e.g. from smaller
fish to larger predatory fish. As a result, nearly 100% of the
mercury that accumulates in
predator fish is methylmercury. Another consequence is that
older fish typically have higher mercury
concentrations in their
tissue than younger fish of
the same species, partly because older fish eat more fish and
larger fish as they age.

The extent to which mercury will
bioaccumulate in any given
situation depends on several factors. One important factor is
how much mercury is converted to
methylmercury, and
vice-versa, particularly by some bacteria in the aquatic
environment. Consequently, the extent of
bioaccumulation and
biomagnification of mercury
in fish remain difficult to predict in specific
situations.

The mercury in fish is passed on to the predators at the top
of the food chain (e.g. humans, seabirds, seals, otters, eagles
and ospreys) and how much they
accumulate depends on the
type and size of fish they eat.
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3.2 How is wildlife affected?

Methylmercury is a poison
for the central nervous system. During the Minamata poisoning
incident in Japan
(see 2.1.1),
birds had trouble flying and behaved abnormally in other
ways.

Mercury can also affect reproduction.
Methylmercury poses a
particular risk to the
developing foetus because it readily moves into the placenta and
can damage the developing nervous system. Mercury may be present
in eggs and harm bird
reproduction even when
concentrations in eggs are
low.

3.3 How may certain ecosystems be affected?

Recent evidence suggests that mercury is reducing
microbiological activity, vital to the terrestrial food chain.
This may already be affecting
forest3
soils over large parts of Europe and potentially in many other
places in the world with similar soil characteristics.

Impacts from long-range transport of mercury in the Arctic
region have been the focus of recent discussion, but the effects
are by no means restricted to this region of the world. The same
food web characteristics
are found in specific
ecosystems and human
communities in many countries, particularly in places where a
fish diet is predominant.